Methylmercury in water, seston, and epiphyton of an Amazonian river and its floodplain, Tapaj´os River, Brazilq

Levels and accumulation of MeHg were characterized in filtered water, suspended organo-mineral matter, phytoplankton, zooplankton and epiphyton during the dry season and the wet season. In open water of the lentic and lotic ecosystems, the MeHg in filtered water -0.2 mm. was near or below the detection limit -0.02]0.03 ngrl.. These concentrations represent -5% of the Hgtot. content in filtered water. Inundated forests Igap´o. and macrophyte floating mats were the only sites where MeHg was significantly detected 0.07]0.24 ngrl., representing 3]22% of the Hgtot. in filtered water. MeHg concentrations in organo-mineral suspended matter 2]26 ngrg dry wt. representing 0.6]7.3% of Hgtot.. were correlated with the N content but not with the C content. Data suggest that MeHg enrichment of suspended matter is strongly influenced by the presence of degraded planktonic remains relatively rich in N and MeHg. In zooplankton, MeHg concentrations 20]140 ngrg dry wt.. increased from the dry season to the end of the wet season. This increase was followed by higher proportions of MeHg during the wet season in comparison to the dry season 15]40 to 50]70% of the Hgtot... The epiphytic material collected from the roots of macrophyte floating mats contained 2]8.5 ngrg dry wt. of MeHg. The proportion of MeHg to Hgtot. in epiphyton 1.5]8.3%. correlated with its C and N contents. The data suggest a greater bioavailability of MeHg in the Tapaj´os River ecosystems due to the seasonal increase in water level and the consequent inundation of the floodplain. Inundation favours the development of large macrophyte floating mats which increases the bioavailability of epiphyton to herbivorousrdetritivorous fish. The root zone of floating macrophytes and the flooded organic horizon of the Igap´o forest are the only sites along the Tapaj´os River where significant MeHg can be detected in the water column and sediment. This new study supports the hypothesis that MeHg production and transfer to the first link of the food chain in Amazonian river systems is closely related to organic matter biogeochemistry in the floodplain environment.